1. Field of the Invention
The present invention generally relates to rate-distortion optimization (RDO) for video coding, and more particularly to a perceptual RDO using structural similarity (SSIM) index as a quality metric.
2. Description of Related Art
The high performance of H.264 video coding is attributed to, among other things, adoption of the rate-distortion optimization (RDO) framework. An objective distortion metric, such as mean square error (MSE), sum of square difference (SSD) or sum of absolute difference (SAD), is conventionally used in the RDO framework. However, these conventional metrics are poorly correlated with perceptual quality because they do not take the characteristics of the human visual system (e.g., visual perception) into account. Further, most conventional video coding systems are disadvantageously designed with such non-perceptual-based metrics.
The object of RDO is to minimize the distortion D at a given rate R by an appropriate selection of the coding modes. The RDO problem is commonly formulated as
      J    =          D      +              λ        ⁢                                  ⁢        R              or      λ    =          -                        ⅆ          D                          ⅆ          R                    where J is a cost function, and λ is the Lagrange multiplier that controls the tradeoff between the distortion and the rate.
As the rate-distortion (R-D) characteristic of the video content cannot be obtained until the video is encoded and the video cannot be encoded unless the R-D characteristic is given, the Lagrange multiplier cannot be directly calculated from the above expression. In order to resolve this dilemma, methods have been proposed but, as known to the inventors of the present application, none of them is both content-adaptive and computationally efficient. Furthermore, most of them are not perceptual-based.
For the reason that conventional methods could not effectively and economically obtain the Lagrange multiplier in order to encode the image, a need has arisen to propose a novel scheme for economically determining the Lagrange multiplier in a perceptual-based manner.